Come autumn, leaves exchange their lush greens for deep reds, but why? Scientists have puzzled over this transformation for over a century. Now a new study of aphids and apple trees reinforces the theory that the red hue wards off insects looking for a leafy snack or a place to nest.

This idea, first proposed by the late British evolutionary biologist William Hamilton, has long been controversial (ScienceNOW[2], 16 July 2001). The theory holds that the leaves' red pigments--conferred by chemicals called anthocyanins--caution insects that the tree is not ideal for dining or nesting. "It's a warning signal, like the bright colors of a [toxic] frog or butterfly," says evolutionary biologist Marco Archetti of Harvard University. But other scientists have suggested that the red pigment defends leaves against sun damage. Because of changes in their physiology, leaves are more susceptible to such damage in the cool autumn weather.

Archetti's new study began when he noticed that red leaves are more common in wild apple trees than in ones grown for fruit. Apples originated in central Asia, and wild trees are still common in countries such as Kyrgyzstan and Kazakhstan. "If you go there in the autumn, you see such bright red colors on the trees.— You don't see that in apple trees in cultivation in Europe," says Archetti. He speculates that domesticated apple trees have lost their autumn "warning" colors because over the centuries farmers crossbred the trees with the largest, tastiest fruit, rather than selecting the ones most resistant to insect pests.

When Archetti compared wild and farmed trees, he found that 62.2% of central Asian wild apple trees had leaves that turn red in the autumn, compared with just 2.8% for cultivated British apple trees. To test the idea that red leaves spell trouble for insects, in the fall of 2007, Archetti placed nesting aphids in red-leaved and green-leaved apple trees. The following spring, 60% of aphids nesting in green trees had survived, compared with only 29% of those in the red trees, he reports today in the Proceedings of the Royal Society B. The reason behind this disparity is unclear, but Archetti's and other studies suggest that the red leaves either have toxic chemical defenses or hold fewer nutrients for young aphids.

Environmental scientist David Wilkinson of Liverpool John Moores University in the United Kingdom calls Archetti's study "imaginative" and predicts it will reinvigorate interest in this colorful debate. But the new study appears unlikely to resolve the dispute. "This work is interesting but should be taken with a sizable pinch of salt," says plant geneticist Andrew Flavell of the University of Dundee in the United Kingdom. He thinks that red leaves may have been lost for another reason--for example, there may be a genetic link between leaf color and fruit taste, he suggests.